Platelet fragments do not contribute to elevated levels of platelet associated IgG

Most assays that measure platelet associated IgG (PAIgG) relate the IgG associated with the test platelets to the platelet count. This could lead to a systematic error if platelet fragments were present in the washed platelet sample but not counted. To address this issue, we studied platelets from patients with idiopathic thrombocytopenic purpura (ITP), thrombocytopenia complicating cardiopulmonary bypass, and laboratory synthesized platelet fragments (freeze-thawed) using electron microscopy. Scanning electron microscopic (SEM) examination of the platelet samples demonstrated appreciable numbers of fragments only in the freeze-thawed specimens. Yet, 'fragments' could be seen in all specimens using transmission electron microscopy (TEM). Most of these 'fragments' proved to be artefacts: we found that the ratio of 'fragments' to intact platelets observed in the TEM specimens was similar to the estimated ratio of 'fragments' to platelets that would have been generated had the specimens been sectioned at 90 degrees to the plane of the actual section. Platelets labelled with the membrane label 125I-iodosulfanilic acid were fragmented by repetitive freeze-thawing. Measurement of pellet radioactivity following washing indicated that the fragments were lost during the washing procedure. These studies indicate that elevated levels of platelet associated IgG in ITP do not represent artefacts due to contamination of the test platelets by platelet fragments.